A/C or Heat?

Spring is the most confusing time of the year when I am not sure whether to turn the A/C 'on' or heat 'on'. If I keep the A/C turned on all the time, it becomes very cold in the night and if I keep the heat on all the time, it becomes very warm during the day. So everyday I have to keep changing between heat and A/C at various times depending on how the weather is. I am sure most people here in Michigan go through this at this time of the year unless they have something to automatically switch between heat and cold depending on outside temperature.